Problem: What number could replace $r$ below? $\dfrac{1}{4} = \dfrac{r}{12}$
Solution: The fraction on the left represents 1 out of 4 slices of a rectangular pie. What if we cut the pie into 12 slices instead? How many slices would result in the same amount of pie? We would need 3 slices. $\dfrac{1}{4} = \dfrac{3}{12}$ and so the answer is $3$ Another way to get the answer is to multiply by $\dfrac{3}{3}$ $\dfrac{3}{3} = \dfrac{1}{1} = 1$ so really we are multiplying by 1. The final equation is: $\dfrac{1}{4} \times \dfrac{3}{3} = \dfrac{3}{12} $ so our answer is $3$.